Field of the Invention
The invention relates to an arrangement for multiplexing and/or demultiplexing the signals of a plurality of optical data channels of different wavelengths that are transmitted in a waveguide.
In optical communications technology, for transmitting the greatest possible quantity of data via an optical waveguide, it is known to multiplex the data to be transmitted. One way of doing this is to transmit information at multiple wavelengths independently and simultaneously over one waveguide. To this end, it is necessary on the transmission side for the signals of the various light sources to be united by an optical multiplexer in an optical waveguide, and on the receiver side for the signals at various wavelengths from the arriving waveguide to be split up by an optical demultiplexer into individual data channels for separate detection.
One way of achieving multiplexing or demultiplexing is to separate the wavelengths of the various data channels using interference filters. Especially at relatively great wavelength spacings between the individual data channels of 10 nm and more, this proves to be quite effective. By means of a high number of interference layers at the individual interference filters, very steep spectral edges between transmission and reflection of various wavelengths can be created. By cascading such filters with individually different spectral transmission and reflection layers, a selection or unification of many wavelength channels can be accomplished. The use of interference filters for achieving multiplexing or demultiplexing is described for instance in European Patent Disclosures EP-A-0 877 264 and EP-A-1 004 907.
In EP-A-0 877 264, the cascading of the filters is done in a parallel optical beam path, which disadvantageously requires beam shaping by means of lenses or mirrors. In EP-A-1 004 907, the use of a divergent beam is described. However, these divergent beams are assigned focusing mirrors, so that once again beam shaping by mirrors is required.
Another possible way of cascading the filters is to guide the light in waveguides. Arrangements are known in which the light guided in a waveguide is reflected at an angle at a mirror surface designed to be wavelength-selective and is carried onward after the reflection in a further waveguide. The requisite cascading can be done with zigzag guidance of the waveguides between a plurality of mirrors.
The object of the present invention is to furnish an arrangement for multiplexing and/or demultiplexing the signals of a plurality of optical data channels that makes it possible in a simple way to achieve multiplexing and demultiplexing of various wavelengths guided in one waveguide. A method for producing such an arrangement is also to be furnished.
Accordingly, the arrangement according to the invention for multiplexing and/or demultiplexing is distinguished in that the waveguide is disposed or embodied in a planar substrate that has a plurality of cutouts extending at a defined angle that interrupt the waveguide at successive points. One wavelength-selective mirror is disposed in each of the cutouts, and at each mirror light is coupled in or out obliquely to the surface of the substrate.
Thus according to the invention, incoupling and outcoupling of light is done in a plane that is at an angle to the plane of the substrate in which the waveguide is disposed. The invention makes it possible to realize the assembly in a simple way, since all that needs to be done is to make lateral cutouts into the substrate at a defined angle from the substrate edge using a wafer saw and then to place the wavelength-selective filters in these cutouts.
In a preferred feature of the invention, all the cutouts have the same angle relative to the substrate surface. This makes simple production of the arrangement possible by sawing or etching the substrate, always at the same angle. In principle, however, the various cutouts can also have different angles.
The angle of the cutouts or wavelength-selective mirrors to the substrate surface is 30, for example, but can be varied over a relatively wide range. Care must merely be taken that the wavelength-selective layers of the mirror, at the angle of incidence set, furnish a steep spectral edge between transmission and reflection of the various wavelengths, so that the individual channels can be cleanly separated from one another or joined together.
The various mirrors are preferably positioned in the various cutouts by way of an adhesive, and the adhesive preferably has a similar or identical coefficient of diffraction to the waveguide of the substrate. It should be noted that by interrupting the waveguide at the cutouts, irregularities can occur at the waveguide. In particular, the end faces of the waveguide are not polished at the interruption points. By using an adhesive that has a similar coefficient of diffraction to the waveguide, irregularities that have formed at the end faces of the waveguide where it has been cut apart are filled up and optically balanced again.
The waveguide used is preferably an optically integrated waveguide. However, it is also within the scope of the invention to use a fiber, in particular glass fiber, as a waveguide that is placed in a suitable indentation in the substrate and secured therein. As examples of materials for the waveguide, glass and plastic can be named. It is also possible to form the waveguide of a semiconductor material; in that case, the waveguide is embodied as optically integrated.
In a preferred feature, especially when the arrangement is used as a receiver or in other words as a demultiplexer, the waveguide disposed in the substrate is embodied as a many-mode waveguide. As a result, the arrangement can be used both in systems with single-mode fibers and in systems with multi-mode fibers.
Multi-mode waveguides have the property that radiation is guided in different directions in the waveguide. This leads to a widening of the spectral admission curve of the interference filters. To counteract this tendency, one advantageous feature of the invention provides using semiconductor materials for the filters or wavelength-selective mirrors. Semiconductor materials have a very high coefficient of diffraction, as a result of which the angular dependency of the filter curve of interference filters can be reduced markedly. Another advantage of using semiconductor materials as filters is that such filters can be produced with very high precision, for instance epitaxially, and by way of example as a material system of GaAs-AlAs. The high coefficient of diffraction of semiconductor materials also lessens the widening of the beam on passing through the filter substrate, which minimizes optical losses upon passage through the filter.
In further preferred feature of the invention, the light coupled in or out via a wavelength-selective mirror is coupled with a respective transmitter or receiver unit which is disposed directly on the substrate. The substrate thus serves additionally for mounting the transmitter or receiver units. By mounting the transmitter or receiver units directly above the radiation exit or entrance region of the surface of the substrate, good optical coupling is assured. The electrical bonding of the transmitter or receiver units is done via a conductor track structure on the substrate, which can be disposed thereon in a manner known per se.
In a preferred refinement of the invention, guide structures for attaching optical waveguides are integrated with the substrate. If silicon is used as the substrate, then the structures can be created with high precision, for instance by anisotropic etching. The provision of such guide structures enables adjustment-free attachment of the multiplexing/demultiplexing assembly to the light-carrying fibers.
In a preferred feature, the waveguide extends near the edge of the substrate. As a result, the lateral cutouts in the substrate can be embodied as relatively short and can be made simply.
In addition to an embodiment of the substrate of silicon, an embodiment of the substrate by plastic is also conceivable in particular. For producing a waveguide in the plastic substrate, the substrate is provided, for instance by embossing, with channel-like structures that are then filled with waveguide material.
The method of the invention for producing a multiplexer/demultiplexer arrangement provides that at a defined angle, cutouts are made into the planar substrate and then wavelength-selective mirrors are placed in these cutouts; the cutouts and wavelength-selective mirrors cut apart the wavelength guide into a plurality of points located one after the other. Fastening of the mirrors in the cutouts is preferably done by means of an adhesive.
In one feature of the method, the cutouts are made in the substrate by sawing using a wafer saw. Since the sawn face has a certain roughness, it is preferably provided that the sawn face be chemically smoothed or etched, for instance using a polish. This has a smoothing effect, which also smooths the end face of the interrupted waveguide.
However, the cutouts can also be made in other ways in the substrate. For instance, in a further feature of the method, the cutouts are made in the substrate by etching in an oblique direction. The advantage of using an etching process is that smooth side walls are automatically present in the cutout.
The method of the invention makes it possible in a simple way to achieve a multiplexer/demultiplexer arrangement.